RFR: JDK-8071767 Improve names and dependencies for image targets

[Ingemar Aberg]

Some of the target names in the makefiles are inconsistent and does not 
clearly reflect what they do. They should be improved but the old names 
should be kept as aliases for people who are used to them.

Examples:

images -> product-images
docs -> docs-image

Bug: https://bugs.openjdk.java.net/browse/JDK-8071767
Webrev: 
http://cr.openjdk.java.net/~ihse/JDK-8071767-improved-global-make-targets/webrev.01/

Thaks Magnus for publishing this webrev for me.

/ingo